Existing blue and green LEDs adopt GaN-based III-V component semiconductor material. Most light is transmitted from the P-type layer due to small hole concentration of P-GaN layer of the GaN-based LED epitaxial wafer and thin P-type layer. P-type layer's inevitable absorption of light results in low LED chip external quantum efficiency and greatly decreases the LED light-emitting efficiency. Adoption of ITO layer as the current spreading layer can enhance the transmittance. However, the LED voltage is high and shortens the service life. In addition, under impressed voltage, the LED service life is shortened due to uneven current diffusion, which leads to large current density in some areas. In short, the existing GaN-based LED has relatively low external quantum efficiency (EQE) due to the non-uniform current distribution and the electrode's absorption of light reflected on it.
Therefore, many studies are made on improving LED's light-emitting efficiency, mainly concerning such technologies as pattern substrate technology, distributed current blocking layer (i.e., current blocking layer), distributed Bragg reflector (DBR) structure, transparent substrate, surface roughening, photonic crystal technology, etc.
Referring to FIG. 1, a conventional LED structure, comprising a substrate 100, a bottom-up-laminated N-type layer 101, a light-emitting area 102, a P-type layer 103, a metal reflecting layer 104, a current spreading layer 105, a P electrode 106 and an N electrode 107 on the exposed surface of the N-type layer 101. The metal reflecting layer 104 (typically, Al or Ag material) reflects part of light emitted by the light-emitting layer and extracts light sideways, as shown in Light 1a. However, it may still be impossible or difficult for some light exit from the side or above, as shown in Light 1b, making the light emitted from light-emitting layer not efficiently extracted, resulting in loss of light and affecting the light-emitting efficiency of the chip.